# Stress in a beam

1. Jan 10, 2013

### Victor.wong

if you look at the attached photo.

we have 2 beams, 1) is shorter, 2) is longer. cross-sectional area is the same. same force is applied X cm from the fixed end.

what is the max stress in each case? why are they the same ( and why not)?

Take gravity out of the equation.
FEA shows differnt result.

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2. Jan 10, 2013

### Studiot

Hello victor, is this a lab experiment you are writing up?

You surely don't need a finite element analysis to distinguish between types of stress, this is more fundamental than FEA.

So what do you mean by max stress?

Do you know how to derive stress as a function of distance?

3. Jan 10, 2013

### AlephZero

How different is "different"? Beam theory is only an approximation. A 3-D finite element model is a different approximation. You wouldn't expect them to be "exactly" the same.

4. Jan 10, 2013

### Victor.wong

I know it is a very simple question. but FEA is telling me sth i dont expect. so ask this equation and see who can explain.

first of all, stress only caused if a block of material is under 2 force. action force and reaction (the fixture). since the length under the point of force of action is not under another reaction force. these length should not affect the stress produced by this action force

in both cases, max stress appeared at the area around the top fixing point. if i let the applied force and location constant, changing the length of the block will change the max stress at the fixing area. why??

5. Jan 10, 2013

### Studiot

Well what do you mean by max stress?

Or if you like, what type of stress?

And how about answering my other questions, they were intended to help.

6. Jan 10, 2013

### Victor.wong

OK. may be a pic will explain more

if you see the new attachment, my question is, why is the length of the block below the applied force affect the stress at the circled area.

my thinking behind is, no matter how long the rod is below the force application. it should not affect the portion on top of the applied force.

right? and why?

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7. Jan 10, 2013

### Studiot

Do you know the difference between bending stress and shear stress?